"A real constant is an integer constant, possibly followed by a point (.) and one or more digits, possibly followed by an exponent symbol E and an integer constant; at least one of the optional parts must occur, hence no integer constant is a real constant. Examples: 0.7, +3.32E5, 3E~7 . Non-examples: 23, .3, 4.E5, 1E2.0 ."
from: Definition of Standard ML Version 2 [Robert Harper, Robin Milner, Mads Tofte] 1988
Update:
The Definition of Standard ML (Revised) 1997 modifies the passage to:
an exponent symbol (E or e ) and an integer constant in decimal notation;